I think brrt is talking about "burning" nuclear waste in a molten-salt reactor or similar. Some types of existing fission reactors can only utilize a small percentage of the uranium in their fuel rods, after which the rods must be removed. The rods must then be stored, and possibly reprocessed afterwards.

MSR and other fission reactor types can utilize 100% of the fissionable material put into them, so this sort of leftover "waste" is not such a problem.